We are continuing to investigate the properties of cells resistant to transformation by specific retroviral oncogenes. New methods for the selection and study of resistant cells have been developed. This includes the use of new mutagens and new selective procedures. Following re-infection of the original resistant cell lines with Ha-MuSV, dot blot analysis shows that resistant cells do not block v-ras mRNA synthesis but must be altered at some point closer to the transformation event. New methods for developing resistant cell have been worked our as have new selective agents. We are currently trying to isolate more resistant cells using these methods. We are studying a new murine virus that induces solid tumors in mice. We have worked with this isolate in cell culture where it transforms cells and appears to be defective. Initial dot blot analysis indicates that this isolate is related to v-Ha-ras.